The first period of construction began in 1173 and
lasted five years. A century later, during an equally
brief bout of construction activity, the tower started to
lean to the south and masons began to build a backward
curve into the stonework in an effort to keep it
vertical. Another century passed before a final decade of
construction completed the belfry in 1370, when the lean
was approximately 3.5 degrees from the vertical.

The 142-tonne Pisa tower itself was likely on the
verge of collapse. The tower had a negligible factor of
safety against toppling and was leaning more each year.
Built on 5 m of silty soil over alternating layers of
clay and sand, the tower would probably have collapsed
during construction.